1. Field of the Invention
The present invention relates to an acceleration sensor which employs piezoelectric element(s) adapted to detect vibration in an automobile or the like.
2. Prior Art
A piezoelectric type sensor has been used for detecting vibration in an automobile. This piezoelectric sensor is described for example in "Force and Acceleration Sensor (1)" p. 81-85, by Keras, Series No. 5, Ceramic Sensor, 1973, No. 32 Ele-Cera Publishing Co.
An acceleration sensor employing such a piezoelectric sensor as described above, a weight, and a charged amplifier is applied for example to an electric control suspension means adapted to control vibration in an automobile in order to improve the driving and riding feel.
In the case that the piezoelectric sensor is applied to the electronic control suspension means, since the piezoelectric sensor has a high impedance and is capacitive, coping with external noise is a problem.
Accordingly, there have been proposed such measures as to incorporate an impedance conversion circuit, and to ground one of the electrodes of a piezoelectric sensor to a casing so that the casing provides a shielding effect.
However, in this instance, in such an application as an automobile in which a single battery power source is available for activation and the body of the automobile is grounded, if one of the electrodes of a piezoelectric sensor is grounded to the casing, a signal for detecting acceleration is variable mainly relative to the ground level, while an amplifier is caused to amplify the signal on the basis of the DC level obtained by dividing by two the source voltage. Under this condition, it is difficult to separate the variation in the source voltage from the detected acceleration signals, and therefore the signals are disturbed by the effects of source voltage variation as well as external noises.
In order to cope with these problems, one solution is to replace the impedance conversion circuit with a charge amplifier. It is to be pointed out, however, that such a charge amplifier is practically difficult to adapt even in a general application which is provided with positive and negative power sources and therefore difficult technically to incorporate in general.
Furthermore, in such an automobile application it has been a problem to provide a circuit which enables the charge amplifier to be activated by a single power source and be made stable against fluctuations in the source voltage.
In addition, it is necessary to float the opposite electrodes of the piezoelectric sensor from the grounding potential in order to deploy the charge amplifier most effectively. Particularly when the casing is grounded, it is a question how to fix a piezoelectric sensor to the casing in an insulated condition.
Further, when an acceleration meter employing a piezoelectric sensor and a weight is used in an automobile, it is necessary to cancel by some means the pyroelectricity, or the characteristic that the piezoelectric sensor generates a charge depending on the change of temperature, in order to pick up only acceleration signals stably regardless of a change in the temperature of the environment.
Furthermore, when the piezoelectric sensor is used as installed in an automobile, it is necessary to remove unnecessary signals from those caused by vibration due to the operation of a suspension system, on such as vibration caused by an engine, and resonant frequencies caused by attachment of the piezoelectric sensor.